This invention relates to methods and apparatus for curling lips about the open mouths of flexible-walled stacked containers, particularly those formed from difficult-to-process thermoplastic, synthetic plastic, material such as polypropylene. A number of systems for curling or beading the rims of thermoplastic containers have been proposed and employed over the years and are exemplified in the following patents:
U.S. Pat. No. 3,096,546 PA1 U.S. Pat. No. 3,337,919 PA1 U.S. Pat. No. 3,339,005 PA1 U.S. Pat. No. 3,358,331 PA1 U.S. Pat. No. 3,487,139 PA1 U.S. Pat. No. 3,355,536 PA1 U.S. Pat. No. 3,579,737 PA1 U.S. Pat. No. 3,676,543 PA1 U.S. Pat. No. 3,861,847 PA1 U.S. Pat. No. 3,910,744 PA1 U.S. Pat. No. 3,949,045 PA1 U.S. Pat. No. 4,048,781
While such systems have proven useful in curling the rims or brims of some plastic containers, they have not, to our knowledge, been sufficiently versatile to handle also containers formed from difficult-to-process materials such as polypropylene. While a suitable brim curling temperature for polypropylene containers may be stated to be generally in the range of 300.degree. to 320.degree. F., the actual range for specific polypropylene materials is normally in the area of plus or minus 5.degree. F. within that general range. For example, with a certain batch of polypropylene which was tested, satisfactory results could be achieved if the brims, at the time of forming were held between 300.degree. and 305.degree. F.
With systems such as disclosed in U.S. Pat. Nos. 3,339,005 3,355,536 and 3,337,919, which employ radiant heat in advance of the forming rolls, the usual result with material such as polypropylene was that the containers were rendered too pliable to properly enter the passageway between the rolls. In other words, sufficient control of the temperature of the container side wall sections, and the temperature of the axial return (brim) sections of the containers, simply could not be achieved to permit the proficient forming of turned-in lips, when the material was a temperature-critical thermoplastic material which requires more heat input relative to surface area such as polypropylene.
Similarly, systems such as disclosed in Edwards U.S. Pat. No. 3,096,546, which utilize only internal roller heating to conductively heat container rims, could not process polypropylene containers efficiently and required rollers of undue length, which were expensive and cumbersome. In contradistinction to the methods previously suggested and commercially employed, the present system utilizes a combination of radiative and conductive heating to bring the container brims quickly, and yet in a controlled manner, to the critical forming temperature, while the rims are engaged in and mechanically supported by, the roller helical grooves. Prior to forming, the radial distance between the brim and the peripheral body wall portion at the mouth of each container is fixed and then, after forming by subjecting the brim to axially opposed pressures, heat is removed conductively by the rollers in a rim cooling zone and while the fixed radial distance mentioned continues to be maintained, until the temperature of the container and brim is lowered sufficiently so that the plastic is set in mechanically stable configuration.
One of the prime objects of the present invention is to provide a versatile apparatus which will operate on difficult-to-form materials such as polypropylene, as well as on other materials, to automatically roll in the edges on plastic containers in controlled, fast, and repetitively trouble-free manner.
Another object of the invention is to accomplish the objects of the invention with an efficient low cost machine having thermally conductive rollers which can be heated efficiently to a critical temperature and which then will form and cool the brims of stacked containers in a manner such that rollers of relatively short length can be employed, even with difficult-to-process thermoplastic materials.
Still another object of the invention is to provide a progressive curling machine of the character described which utilizes a plurality of continuous helical grooves which are capable of processing containers such as beverage cups and cottage cheese containers at higher rates of speed than previously.